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Does the debian kernel sends the gratuitous arp ?

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Balaji G

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Oct 6, 2023, 8:40:06 AM10/6/23
to
Hi,
    I am using "Debian GNU/Linux 11 (bullseye)" with kernel version 5.16.12.
When i do a link up/down i don't see any Gratuitous ARP being sent.

# echo 1 > /proc/sys/net/ipv4/conf/eno5np0/arp_notify
# ip link set down dev eno5np0
# ip link set up dev eno5np0

Captured all the packets via tcpdump & the tcpdump is not showing any Gratuitous ARP packets.

But, with the same commands i could see the Gratuitous ARP being sent in Red hat.9.0 (Plow). 
So, please let me know if this is a specific scenario in Debian 11 ??

Thanks,
Balaji







Geert Stappers

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Oct 6, 2023, 3:20:06 PM10/6/23
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On Fri, Oct 06, 2023 at 05:52:16PM +0530, Balaji G wrote:
> Hi,
> I am using "Debian GNU/Linux 11 (bullseye)" with kernel version 5.16.12.
> When i do a link up/down i don't see any Gratuitous ARP being sent.

ARP


> # echo 1 > /proc/sys/net/ipv4/conf/eno5np0/arp_notify

Probably transmitted "I tried to enforce ARP notification"


> # ip link set down dev eno5np0
> # ip link set up dev eno5np0

link


> Captured all the packets via tcpdump & the tcpdump is not showing any
> Gratuitous ARP packets.

Why should it? (Yes, that is a serious question.)


> But, with the same commands i could see the Gratuitous ARP being sent in
> Red hat.9.0 (Plow).

I think more factors have been change as kernel.


> So, please let me know if this is a specific scenario in Debian 11 ??

I state that the OP, Original Poster, is the specific scenario.
Yeah, that is a blunt statement. Now hear me out.

Gratuitous ARP is "linking" MAC-address and IP-address.

During `ip link set down dev eno5np0` and `ip link set up dev eno5np0`
are NO IP-addresses involved.

So there is no need for ARP.


I do hope to see a follow-up message like:

Re-did the test with a static IP-address on the interface
and indeed see a gratuitous ARP

or some thing like

To reproduce the missing gratuitous ARP do ...

Yeah, that might reveal more information about dev eno5np0.


> Thanks,
> Balaji

Groeten
Geert Stappers
--
Silence is hard to parse

Jeffrey Walton

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Oct 6, 2023, 5:30:07 PM10/6/23
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I think that's now Poettering:
https://github.com/systemd/systemd/blob/main/src/libsystemd-network/sd-ipv4acd.c#L302

Jeff

to...@tuxteam.de

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Oct 7, 2023, 1:40:07 AM10/7/23
to
As Geert says, probably it needs an ip address to be able to send
an ARP. Perhaps the Redhat box has one set by default?

What does "ip addr show dev xxxx" say?

Cheers
--
t
signature.asc

Jeffrey Walton

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Oct 7, 2023, 12:00:07 PM10/7/23
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OP asked if this was specific to Red Hat. The answer is No, it is
baked into Systemd nowadays.

> What does "ip addr show dev xxxx" say?

I have no idea.

Jeff

Balaji G

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Oct 8, 2023, 2:10:06 AM10/8/23
to
Hi Jeff,
           Do you mean this is a known issue & will be fixed in the future releases ?




Thanks,
Balaji

Geert Stappers

unread,
Oct 8, 2023, 5:00:07 AM10/8/23
to
On Sun, Oct 08, 2023 at 11:21:10AM +0530, Balaji G wrote:
> On Sat, 7 Oct 2023 at 02:50, Jeffrey Walton wrote:
> > On Fri, Oct 6, 2023 at 2:04 PM Balaji G wrote:
> > >
> > > Hi,
> > >
> > > I am using "Debian GNU/Linux 11 (bullseye)" with kernel version 5.16.12.
> > > When i do a link up/down i don't see any Gratuitous ARP being sent.
> > >
> > > # echo 1 > /proc/sys/net/ipv4/conf/eno5np0/arp_notify
> > > # ip link set down dev eno5np0
> > > # ip link set up dev eno5np0
> > >
> > > Captured all the packets via tcpdump & the tcpdump is not showing any
> > > Gratuitous ARP packets.
> > >
> > > But, with the same commands i could see the Gratuitous ARP being sent in
> > > Red hat.9.0 (Plow).
> > >
> > > So, please let me know if this is a specific scenario in Debian 11 ??
> >
> > I think that's now Poettering:
> >
> > https://github.com/systemd/systemd/blob/main/src/libsystemd-network/sd-ipv4acd.c#L302
> >
> > Jeff
> >
> Hi Jeff,
>
> Do you mean this is a known issue & will be fixed in the future releases ?
>

Here not Jeff. What I do understand from Jeff message is
something like: "I can bitch on systemd, so I do"
Time will tell how much Jeff contributes in solving
the interesting problem of the invisible gratuitous ARP.

Meanwhile lets follow-up
on https://lists.debian.org/debian-user/2023/10/msg00181.html


> Thanks,
> Balaji

You might have an extra copy of this message.
Because I did this time "reply-to-all" insteadof "reply-to-list".

The good thing of "reply-to-list" is that it reveals who reads
mailinglist postings. Yes, I'm trying to tell that you
missed https://lists.debian.org/debian-user/2023/10/msg00181.html


Regards

Geert Stappers

unread,
Oct 8, 2023, 5:20:06 AM10/8/23
to
On Fri, Oct 06, 2023 at 09:19:32PM +0200, Geert Stappers wrote:
> On Fri, Oct 06, 2023 at 05:52:16PM +0530, Balaji G wrote:
> > Hi,
> > I am using "Debian GNU/Linux 11 (bullseye)" with kernel version 5.16.12.
> > When i do a link up/down i don't see any Gratuitous ARP being sent.
>
> ARP

Address Resolution Protocol


> > # echo 1 > /proc/sys/net/ipv4/conf/eno5np0/arp_notify
>
> Probably transmitted "I tried to enforce ARP notification"
>
>
> > # ip link set down dev eno5np0
> > # ip link set up dev eno5np0
>
> link

As in "the link layer"


> > Captured all the packets via tcpdump & the tcpdump is not showing any
> > Gratuitous ARP packets.
>
> Why should it? (Yes, that is a serious question.)

As opening question for further discussion.

Because I like the question
Does the Debian kernel sends the gratuitous ARP ?

I think the answer is YES
and I very curious when gratuitous ARP is ommitted.


> > But, with the same commands i could see the Gratuitous ARP being sent in
> > Red hat.9.0 (Plow).
>
> I think more factors have been change as kernel.

As invitation for telling how the comparsion was done.


> > So, please let me know if this is a specific scenario in Debian 11 ??
>
> I state that the OP, Original Poster, is the specific scenario.
> Yeah, that is a blunt statement. Now hear me out.
>
> Gratuitous ARP is "linking" MAC-address and IP-address.
>
> During `ip link set down dev eno5np0` and `ip link set up dev eno5np0`
> are NO IP-addresses involved.
>
> So there is no need for ARP.
>
>
> I do hope to see a follow-up message like:
>
> Re-did the test with a static IP-address on the interface
> and indeed see a gratuitous ARP
>
> or some thing like
>
> To reproduce the missing gratuitous ARP do ...
>
> Yeah, that might reveal more information about dev eno5np0.
>
>
> > Thanks,
> > Balaji
>
> Groeten
> Geert Stappers

Because in https://lists.debian.org/debian-user/2023/10/msg00212.html I
wrote "Lets follow-up
https://lists.debian.org/debian-user/2023/10/msg00181.html "

Jeffrey Walton

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Oct 8, 2023, 11:30:06 AM10/8/23
to
On Sun, Oct 8, 2023 at 4:58 AM Geert Stappers <stap...@debian.org> wrote:
>
> On Sun, Oct 08, 2023 at 11:21:10AM +0530, Balaji G wrote:
> > On Sat, 7 Oct 2023 at 02:50, Jeffrey Walton wrote:
> > > On Fri, Oct 6, 2023 at 2:04 PM Balaji G wrote:
> > > >
> > > > Hi,
> > > >
> > > > I am using "Debian GNU/Linux 11 (bullseye)" with kernel version 5.16.12.
> > > > When i do a link up/down i don't see any Gratuitous ARP being sent.
> > > >
> > > > # echo 1 > /proc/sys/net/ipv4/conf/eno5np0/arp_notify
> > > > # ip link set down dev eno5np0
> > > > # ip link set up dev eno5np0
> > > >
> > > > Captured all the packets via tcpdump & the tcpdump is not showing any
> > > > Gratuitous ARP packets.
> > > >
> > > > But, with the same commands i could see the Gratuitous ARP being sent in
> > > > Red hat.9.0 (Plow).
> > > >
> > > > So, please let me know if this is a specific scenario in Debian 11 ??
> > >
> > > I think that's now Poettering:
> > >
> > > https://github.com/systemd/systemd/blob/main/src/libsystemd-network/sd-ipv4acd.c#L302
> > >
> >
> > Do you mean this is a known issue & will be fixed in the future releases ?
>
> Here not Jeff. What I do understand from Jeff message is
> something like: "I can bitch on systemd, so I do"
> Time will tell how much Jeff contributes in solving
> the interesting problem of the invisible gratuitous ARP.

Nope, not that.

I learned a long time ago to answer the question that was asked in the
body of the post.

You're welcome to complain about Systemd all you like. You won't get
any complaints from me. It's not a discussion I will engage in.

Jeff

debia...@howorth.org.uk

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Oct 8, 2023, 1:30:06 PM10/8/23
to
If you'll forgive me stepping in ...

I suspect neither Geert's nor Balaji's first language is English, and
independently notice that Geert writes in a somewhat terse style whilst
Balaji omits potentially useful information (such as the actual tcpdump
output). So I'll add my summary of what I understand:

Gert is pointing out that ARP packets are sent in connection with
resolving the IP address associated with a MAC address, and that
nothing that Balaji has posted suggests that an IP address has been
allocated yet, so there would be no reason for a Gratuitous ARP packet
to be sent.

So he is interested to learn how the IP address is set up on both the
Debian system and the Red Hat system (static or DHCP or something
else?) and I suspect would ideally like to see sufficient message logs
to demonstrate the truth of the answers.

Specifically, on the Debian system is the IP address set statically,
and are there any packets containing the IP address in the logs? Either
transmitted or received. Similarly for the Red Hat system but perhaps
less important.

> Groeten
> Geert Stappers

Balaji G

unread,
Oct 9, 2023, 11:30:05 AM10/9/23
to 
Hi Geert,
 >>> During `ip link set down dev eno5np0` and `ip link set up dev eno5np0`
>>>>> are NO IP-addresses involved.
This interface eno5np0 is configured with the ip addresses(10.45.10.4 ).
# ifconfig
eno5np0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 10.45.10.4  netmask 255.255.255.0  broadcast 10.45.10.255
        ether 88:e9:a4:49:18:48  txqueuelen 1000  (Ethernet)
        RX packets 2967346895  bytes 4417546189260 (4.0 TiB)
        RX errors 0  dropped 1  overruns 0  frame 0
        TX packets 1634521866  bytes 2220475762246 (2.0 TiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0



Thanks,
Balaji

Geert Stappers

unread,
Oct 9, 2023, 4:30:05 PM10/9/23
to
On Mon, Oct 09, 2023 at 08:39:02PM +0530, Balaji G wrote:
> Hi Geert,

Hello mailinglist (archive )readers of debia...@lists.debian.org


> >>> During `ip link set down dev eno5np0` and `ip link set up dev eno5np0`
> >>> are NO IP-addresses involved.
>
> This interface eno5np0 is configured with the ip addresses(10.45.10.4 ).
>
> # ifconfig
} ( mismatch between `ifconfig` command expected `ifconfig` output )
> eno5np0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
> inet 10.45.10.4 netmask 255.255.255.0 broadcast 10.45.10.255
> ether 88:e9:a4:49:18:48 txqueuelen 1000 (Ethernet)
> RX packets 2967346895 bytes 4417546189260 (4.0 TiB)
> RX errors 0 dropped 1 overruns 0 frame 0
> TX packets 1634521866 bytes 2220475762246 (2.0 TiB)
> TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0


I did missed "LOWER_UP", then I noticed that `ifconfig` is used. More
on that later. Back to 'Subject: Re: Does the debian kernel sends the
gratuitous arp ?'

To me is the answer "Yes, Debian kernel sends gratuitous ARP when it
supposed to do". But we, this mailinglist, have this discussion because
somewhere is the gratuitous ARP missing. What triggered the "I wonder
why there is no gratuitous ARP?" is yet unknown.

Let me try to capture a gratuitous ARP. I will be using only one
interface (it is OK if the computer has many other network interfaces)
Using two logins as root, one for "tcpdump", the other "set address".

In "tcpdump session"

tcpdump -n -i eth0 arp


In "set address session"

sudo ip address add 198.51.100.23/24 dev eth0


Back to "tcpdump session".

And to my surprise: **no** gratuitous ARP !

It should be there. I even tried after:

$ cat /proc/sys/net/ipv4/conf/eth0/arp_notify

0
$ echo 1 | sudo tee -a /proc/sys/net/ipv4/conf/eth0/arp_notify
1
$ cat /proc/sys/net/ipv4/conf/eth0/arp_notify
1
$


I'm gonna sleep about. Expect me back over 48 hours.



> Thanks,

Yeah, thanks for the good question.


> Balaji



Groeten
Geert Stappers


About ifconfig :
https://serverfault.com/questions/633087/where-is-the-statement-of-deprecation-of-ifconfig-on-linux

Balaji G

unread,
Oct 10, 2023, 12:10:05 AM10/10/23
to
Hi Groeten,
                The IP address is already assigned to this interface device eno5np0. Please find my reply mail below.


Thanks,
Balaji

Geert Stappers

unread,
Oct 11, 2023, 4:20:06 PM10/11/23
to
On Mon, Oct 09, 2023 at 10:15:38PM +0200, Geert Stappers wrote:
> On Mon, Oct 09, 2023 at 08:39:02PM +0530, Balaji G wrote:
> > Hi Geert,
>
> Hello mailinglist (archive )readers of debia...@lists.debian.org
>

Hello again,
> I'm gonna sleep about it. Expect me back over 48 hours.

:-)


> > Thanks,
>
> Yeah, thanks for the good question.
>

It did learn me that things have changes.
I hope it is the improvement that was aimed for.

No, I have no idea if the H.A., High Available, setup I did build
for a previous employer are effected.

> > Balaji
>
> Groeten
> Geert Stappers
>
>
> About ifconfig :
> https://serverfault.com/questions/633087/where-is-the-statement-of-deprecation-of-ifconfig-on-linux
>

a.k.a. use `ip addres`, `ip a` for short.


Groeten
Geert Stappers
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